Radio apparatus



N. B. THARP RADIO APPARATUS May 22, 1951 2 Sheets-Sheet 1 Filed March 13, 1948 f'cfaas) ml INVENTOR. /Ve/son Thorp.

Patented May 22, 1.951

UNITED STATES OFFICE RADIO. APPARATUS Nelson- B; Tharp, Ellicott City, Md., assignor to Westinghouse Electric Corporation, East Pitts.- burgh,. Pa., acorporation of Pennsylvania. Application MarchlS, 1948, SerialNo, 14,793

(Cl. 33-2w38).

6 Claims. I This invention relates to audio frequency, vacuum tube amplifiers; and relates more pan` ticularly to modulators forradio transmitters havingY class B- output stages'.v

Itis desirable to operate class B modulatortubes, as used in radio transmitters, with as lowstatic plate currentsV as possible so thatthe power dissipatedl therein, under no and low modulation conditions, will be as small-as possible. However; due to the characteristics of the tubes where theycut-oi in `plate current, considerable harmonic distortion results, and spurious oscillations are generated at high modulation levels, if the tubesn are run at; or too near cut-oli, as often happens when their plate voltage supply source has poor voltage regulation.

This invention provides for varying the grid bias of a vacuum tube formaintaining the proper relationshipV between the amplitude of the grid biasand plate supply voltages at different signal levels when the plate supply voltage var-ies underload. Forexample, when the grids of class Boutput tubesare driven more positive on a largev signal level increase ony the grids of.' the driver tubes, the resultingV plate current increase will cause the plate supply' vol-tage to decrease when the voltage regu-lation is poor. Utilizingv this invention, the grids of` the-driuzertubev are so biasedthat the same larg-e signal level increase on the grids oi the driver tubes, which causes'- the plate supply voltage to decrease, will cause'the average direct current values or the cathode currents ofthedriver tubes tov increase and flowthrough resistors across which voltages are developed, which voltages cause theefectivegrid bias voltagesof the output tubes to become less negative, thus automatically compensating fory thefall in platevoltage. Thus, at dinerent, relatively high, sig'- nal levels, the grid bias voltages of the output; tubes will be. automatically adjusted so as to maintain the proper relationship between the grid and plate supply voltages.

Anobject of the invention is to` prevent harmonic distortion in thel output of a vacuum tube ampliler having a plate supply source with poor voltage regulation.0

Another object `of the invention is: to prevent the generation, oi; spurious oscillations in a vacuum tube amplier7 having a .plate voltage supply source with poor voltage regulation.

Another IOblect ofl theinvention is to maintain the proper relationship between the grid bias and plate, supplyy voltages of` a vacuum tubev when thev plate supply voltage varies.A underV load.

Another objectgof the.` invention is to use the cathode current increase of a class AB driver tube caused by a large increase` in signalV levelapplied to the grid of the tube, to decrease the negative grid bias voltage of a following tube to which itisconnected.

Y The invention will now lbe'described with refer-- ence to the drawings; ofwhich:

Fig. l is a graph illustrating the .plate currents and grid voltages of a pair oi proper-lyoperated'l class B' output tubes using a plate voltagesuppl-y source having good voltage regu-lation;

Fig. 2 is a graph similarto Fig. 1 buty illustrating the operating conditions when the'plate voltage supply source'has poor voltage regu-lation;

Fig. 3 is a simplified circ-uit schematic illustrating this invention, as` embodied in a modu lator having class B output tubes dri-ven by class AB cathode follower tubes; and:-

Fi'g. 4 is a circuit schematic illustratingl this invention embodied in a class B modulator or a radio transmitter.

Referring rst to Fig.l 1, the continuous line, sine wave curve mar-kedeg I` illustrates the grid voltage of one of the push-pull connected outputtubes of a class B modulator; the continuous line, sine wave cur-ve marked ip #l illustrates' the resultim.;v plate current of the same tube; the dashed-line, sine'wave curve;v marked eg #2 illus,- tratesthe grid voltage of the other output tube; and the dasheddine, sine wave curve marked" ip #-2 illustrates; the resultant plate current, of' the other output tube. The horizontal line marked 2 is a line along which there is no. plate current during the operating` cycles; the hori ZQntaI;` line marked" I, is.u aline. Showing the static, plate current when there, isnor modulation; the horizontal line marked-3.15 the line of maximum grid" voltage; the horizental line marked 4 is the line of zero grid biasY voltage; the horizontal, line. marked 51S the line Qfnormal' grid bias voltage; and.' the horizontal line. marked 6E is the line ot cut-off grid. bias. voltage,` the. bias, voltage where,- the plate current is zero. The vertical dashed. lines marked D represent. the start of each cycle.;

i the vertical dashed lines, markedA. represent the.

times after the Starts, ofk the cycles, where the plate current curves cross the static. current., lines; and the vertical dashed lines marked. Bf represent time intervals following the crossing ci the static currentline by the plate culllerlt 01.1.1.Ivl-2,V the intervals DLA equalling the intervals. A..-`i3.1 The bias'voltage line 5' of Fig.V ,l` is seen, to runfait enough above the cut-ofi` bias voltage line 6 to permit suiiicient` static current i to flow so that. there isa minimum of' harmonic distortion. The

3 tubes draw plate current simultaneously during the intervals D-B.

Fig. 2 illustrates the operation of the same tubes when the plate voltage regulation is poor. The identication of the curves and of the horizontal and vertical lines of Fig. 2 is the same as that of Fig. 1, but it will be observed that the static plate current line I, and the zero plate current line 2 in Fig. 2 coincide under conditions of maximum modulation. From this it can be observed that the cut-off bias voltage line 6 of Fig. 2, has been shifted above the bias voltage line 5. .This represents the condition where, under modulation, the plate supply voltage falls so that the necessary voltage to cut off the flow of plate current is reduced. This causes, as illustrated by Fig. 2, the plate current of the tubes to be cut off during the intervals D-B, a substantial portion of each cycle. As is known, severe harmonic distortion is introduced into the output of such tubes when their plate currents are cut off, or even closely approach cut-01T, during any part of the operating cycle.

Another serious disadvantage of having the plate currents of class B output tubes cut off during portions of their operating cycles is that when the plate current of one tube is cut off and the plate current of the other tube does not begin to rise for an instant after, the leakage reactance of the output transformer causes oscillations in the plate circuits, especially at high frequencies, which oscillations will be fed back into the grid circuits unless some provision is made to damp them out. This condition will also occur when non-linearity of plate current exists in the region where both tubes conduct simultaneously.

A feature of this invention is that the bias voltages of the grids of class B output tubes are varied automatically so that regardless of variations in the plate supply voltage resulting from poor regulation, as the plate current of one tube approaches zero, the plate current of the other tube will have started to rise, thus providing continuous flow of plate current which prevents L the generation of spurious oscillations, and of harmonic distortion in the tube output circuits.

Another feature of this invention is that it enables the tubes of a class B modulator to be operated at relatively low static plate current levels when there is low or no modulation, resulting in high efciency, and without distortion resulting from poor plate voltage supply regulation under high modulation operation.

,In a class AB driver stage as used in this invention, the grids of the driver tubes are so biased that they operate class A at low signal levels, at which time the direct current values of their cathode currents remain substantially constant as in straight class A stages, and that they operate class AB1 or ABZ at high signal levels at which time the average direct current values of their cathode currents increase with increases in signal levels.

The invention now be described with reference to Fig. 3. The class AB driver tubes I have the conventional input circuits consisting of the load impedances I I and the bias voltage supply sources I2 connected between the grids I3 and the cathodes I4. The grids I3 of the tubes I0 are adapted to be connected through the coupling condensers 2| to a suitable amplifier which is not illustrated. The tubes I0 are connected as cathode followers to the class B modulator tubes I5 through the connection of the cathodes I4 of the tubes I0 to the grids I 6 of the tubes I5; through the connection of the cathodes I'I of the tubes I5 to ground; and through the connection of the cathodes I4 of the tubes I0 to ground through the series connections of the inductances I8, the resistors I9, and the bias voltage supply sources 28.

The plus sides of the bias voltage supply sources 20 are connected to ground, and their minus sides are connected in series with the resistors I9 and the inductances I8, to the cathodes I4 of the tubes I0.

The inductances I8 are designed to have low direct current resistance and to have high reactances to the frequencies being used. The signal voltages which are developed across the inductances I8 excite the grids I6 of the tubes I5 through the cathode follower connections. The tubes I5 obtain their negative grid bias voltages through the inductances I8 and the resistors I9, from the bias voltage supply sources 28. The driver tubes I0 are biased to operate class AB so that on large signals, the average direct current, cathode currents passing through the resistors I9 will rise and fall with the modulation envelope. Thus, as the signal level increases in amplitude above the level where the tubes operate class A, the average direct current values of the respective cathode currents increase, and voltages are developed across the resistors I9, the polarities of which are such as to oppose the negative bias voltages supplied by the bias supply sources 20, and so make the effective biases on the grids of the tubes I5 less negative.

When the grids I6 of the tubes I5 are driven more positive on signal level increases, their plate currents will increase, and the voltage of the plate supply source will decrease if its voltage regulation is poor. The same signal level increase will, however, cause the grid biases of the tubes I5 to become less negative, and this will cause the operating points on their characteristic curves to shift so that the proper relations between plate currents and grid voltages for avoiding distortion are maintained.

It is seen, therefore, that this invention permits the modulator tubes to be set at a fairly low static current (near cut-01T) for highly efficient operation during periods of zero or low modulation, and automatically decreases the negative grid bias of the tubes under conditions of high modulation and lower plate voltage, so as to prevent the tubes from operating in the approach to cut-01T, or at cut-ofi, with resulting distortion and oscillations.

Fig. 4 illustrates the invention embodied in a class B modulator for a radio transmitter. The conventional input transformer 38 has a primary winding 3| adapted to be connected to a source of signal currents, and has the secondary winding 32 connected to the input of the conventional voltage amplifier 33, and through the conventional degenerative feed-back connections 34, to the conventional voltage dividing network 35 in the plate circuits of the tubes I5.

The plate 36 of the tubes I5 are connected through the conventional radio frequency chokes 3l to the network 35, and through the conventional resistors 38 to the ends of the primary winding 39 of the conventional output transformer 40.

The output of the voltage amplifier 33 is connected to the grids I3 of the tubes I 8, which are connected through the grid resistors 4I and the capacitors 42 to the center points of the secondary windings 43 of the filament transformers attacco.

from the bias supply sources I2, the alternating current components flowing through the capacitors 42.

The cathodes II of the tubes I5' are energized by the secondary windings 50 of the filament transformers 5I, the center points of the wind'- ings 50 being connected through the conventional ammeters 52 to ground, the ammeters being shunted by the capacitors 54.

An increase in signal level reaching the grids of the driver tubes; I- through the input transformer 30 and the voltage amplier 33, sumcient to cause the tubes to change from class-A toclass AB operation, causes increased average direct current, cathode current to. flow through the inductances I8, the resistors I9, and the bias Supply sources 20 to ground, the negative sides of the plate supply voltage sources, which are not illustrated, being grounded as is conventional. The grids of the output tubes I will bedriven more positive by the driver tubes I il upon the increases in signal level so that the tubes I5 will draw more plate current from their plate voltage supply source causing the plate voltages tc drop, assuming the plate voltage supply source has poor voltage regulation. At the same time, the increased average direct current from the cathodes I4 through the resistors I9 causes voltage drops thereacross which decrease the negative bias volt-r ages on the grids I6 of the tubes I5. This will cause the tubes I5 to draw suliicient plate currents under high modulation conditions, as described in the foregoing in connection with Fig. 3, for avoiding distortion.

By Way of example, each driver tube III may be a WL 473 and each modulator tube I5 may be a WL 5671.

The plate voltage of the driver tubes II) may be 3300 volts, direct current, and their bias supply sources I2 may supply biases of 250 volts, direct current. The cathode current of each driver tube when operating class A, may be 75 milliamperes, direct current. The cathode current of each driver tube when operating class ABl for providing, for example, 100 per cent modulation, may be 200 milliamperes, direct current. Each resistor I9 may have a resistance of 900 ohms. Each inductance I8 may have a value of '75 henries with a low direct current resistance.

Under static conditions, the plate voltage of the modulator tubes I5 may be 15,500 volts, direct current, and their bias supply sources 25 may provide voltages of 450 volts, direct current. The drop through each resist-or I9 under static conditions would be approximately 67 volts, so that the biases supplied to the grids of the modulator tubes at such times would be approximately 382 volts.

At 100% modulation, when the average, direct current values of the cathode currents of each driver tube would increase to 260 milliamperes, the drop through each resistor would increase to 180 volts, resulting in the bias on the grid ofeach modulator tube being decreased triv 270i volts. This will cause the modulator tubes to operate sufficiently above cut-off that no distortion can result.

Thus when the modulator tubes would normally operate with distortion due to poor plate voltage regulation, as illustrated by the operating curves of Fig. 2, this invention through automatically maintaining the proper relationship between the effective plate voltage and the grid bias voltage, of the modulator tubes, will cause them to operate properly as illustrated by the operating curves of Fig. 1.

It is understood that it is within the scope of my invention to operate the driver tubesv class ABZ, ii it is so desired. It also being understood by one skilled in the art that during class ABZ operation the grids of the driver tubes draw current at higher signal levels.

I claim as my invention:

1. A signal amplifier comprising an output stage including a thermionic tube having a grid; a driver stage including athermionic driver tube having a grid and a cathode connected to said output stage; means for biasing the grid of said driver tube so that it operates class A at low signal levels and operates class AB at high signal level's; a bias voltage supply source connected to said output tube grid so as to apply a negative bias voltage to same, and means for utilizing an increase in the direct current flowing through said cathode resulting from an increase in the strength of the signal on the grid of the driver tube suicient to change same from class A to class AB operation, for providing a bias voltage which decreases the nega-- tive bias voltage from said source on said output tube grid.

2. A Vsignal ampliiier comprising an output stage including a thermionic output tube having a grid; a driver stage including a thermionic driver tube having a grid and a cathode, connected to said output stage; means for biasing the grid of said driver tube so that it operates class A at low signal levels and operates class AB at high signal levels; a bias voltage supply source connected to said output tube grid for applying a negative bias voltage to same, and mean including a resistor in series with said source and the output tube grid for utilizing an increase in the strength of the signal on said driver tube grid suflicient to change same from class A to class AB operation, for providing a bias voltage which decreases the negative bias voltage from said source on said output tube grid.

3. A signal amplifier comprising an output stage including a thermionic output tube having a grid and a cathode; a driver stage including a thermionic driver tube having a grid and a cathode; means for biasing the grid of said driver tube so that it operates class A at low signal levels and operates class AB at high signal levels; an inductance connected at one end to said driver tube cathode and to said output tube grid; a bias voltage supply source having its positive side connected to said output tube cathode, and a resistor connected to the negative terminal of said source and to the other end of said inductance, said inductance and said resistor having such values that the signal voltages developed across said inductance excite said output tube grid, and that an increase in the average, direct current, cathode current through said resistor resulting from an increase in the signal level on said driver tube grid sufficient to change same from class A to class AB operation, causes a voltage drop across said i resistor which decreases the negative bias supplied by said source to said output tube grid.

4. A modulator comprising a class B output stage including a thermionic output tube having a grid; a driver stage including a thermionic driver tube having a grid, connected to said output stage; means for biasing said driver tube grid so that it operates class A at low signal levels and operates class AB at high signal levels; means including a bias voltage supply source for maintaining a negative bias voltage upon said output tube grid when no modulating signals are applied to said driver tube grid, and mean eiective when modulating signals having levels high enough to change said driver tube from class A to class AB operation, are applied to the grid thereof, for decreasing the negative bias supplied by said supply source to said output tube grid.

5. A modulator comprising a class B output stage including a thermionic tube having a grid; a driver stage including a thermionic driver tube having a grid and a cathode, connected to said output stage; means for biasing said driver tube grid so that it operates class A at low signals and operates class AB at high signal levels; means including a grid bias voltage supply source for maintaining a negative bias voltage upon said output tube grid during periods when no modulating signals are applied to said driver tube grid, and means including a resistor connected to said cathode and to said output tube grid and in series with said source, for utilizing an increase in the direct current flowing through said cathode resulting from a modulating signal on said driver tube grid having a level sufficient to change same from class A to class AB operation, forV providing a bias voltage which decreases the negative bias from said source on said output tube grid.

6. A modulator comprising a class B output stage including a thermionic tube having a grid and a cathode, a driver stage including a thermionic tube having a grid, a cathode and a plate, a biasing source connected to said grid of said driver tube, said driver tube being biased at low modulation percentages so that said grid draws no current and plate current of said driver tube is continuous over a complete cycle, said driver tube being biased at higher modulation percentages so that said grid draws current and plate current of said driver tube is cut off over a portion of a cycle, and a connection for impressing signal voltage developed in the cathode circuit of said driver tube upon the grid of said output stage. l

NELSON B. THARP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,093,072 Cooper Sept. 14, 1937 2,131,443 Kummerer et al. Sept. 27, 1938 2,198,464 Shepard Apr. 23, 1940 2,266,096 Timmer Dec. 16, 1941 FOREIGN PATENTS Number Country Date 450,036 Great Britain July 9, 1936 OTHER REFERENCES Article, A New High-Eiciency Linear Amplier, by J. A. Hawkins in Radio for May 1936,

Vpages 8-14. 

